Your question brought back memories of past family holiday dinners
when the siblings and I would endeavor to play various songs on Mom's
best wine glasses. We had a lot of fun, and somehow managed not to
ever break any of them :)
So, what made those impromptu performances possible? In a word,
From the "Apple Valley High School Physics Web Page" we learn this:
"Question: What causes the glass to sing when your finger goes around
The ridges in your finger set the glass into vibration at its natural
frequency. These vibrations build and form a loud sound.
Question: Does your fingertip need to be wet? What does the moisture
have to do with anything?
Your fingertip needs to be wet in order to remove the oils from
between your finger's ridges. This helps produce just the right amount
of friction between your fingertip and the glass."
Here's a slightly more detailed explanation from the How Stuff Works
"Every material (such as glass, steel, concrete) has a natural
frequency at which it vibrates, called a resonant frequency. If you
put energy into the substance at its resonant frequency, you will
force it to vibrate or resonate (resonance is a forced vibration). In
the case of the wine glass, your finger slides and sticks along the
surface of the glass as you rub the rim (a wet fingertip has no oil
and makes a better contact with the glass). The rubbing imparts energy
to the glass molecules and causes them to resonate. The motion of your
hand sets up a wave of vibration traveling through the glass. The
vibrating glass causes air molecules to vibrate at the same frequency.
The vibrating air molecules are the sound wave that you hear (the
frequency or pitch of the sound wave is the same as the resonant
frequency of the glass).
So, how does the water change the pitch of the singing wine glass? As
the resonant wave moves around the glass, it drags the water molecules
with it, creating a wave of water that you can see near the edge of
the glass. The dragging water molecules effectively increase the mass
(both the water and the glass molecules) and reduce the energy of the
wave traveling through the glass. When the energy is reduced, so is
the frequency of the wave in the glass, which is reflected in the
pitch of the sound wave that you hear."
Those explanations are educational, but they leave unanswered a
nagging question: why don't regular glasses make music like the good
crystal? Here's an enlightening discussion of that question from "How
Things Work" by Lou Bloomfield:
What is the difference between crystal and glass?
"The 'crystal' that's used in fine glassware is actually a glass, but
it is chemically different from the glass that's used in more common
glassware. Both materials are formed by melting together a mixture of
silicon dioxide (also called quartz or silica) and other chemicals and
both are glasses, meaning that their atoms are arranged haphazardly
and not in the crystalline lattices of such materials as salt or
sugar. But the chemicals that are added to silicon dioxide to make
normal glassware--sodium oxide and calcium oxide--make the glass
easier to melt and work with at the expense of strength.
That's why normal glassware is relatively soft, emitting a dull sound
when you rap it because it experiences lots of internal friction. In
contrast, the chemicals added to silicon dioxide to make "crystal"
glassware include lead oxide, which makes the glass easier to melt but
doesn't weaken the glass nearly so much. Lead "crystal" glassware is
relatively hard and emits a ringing tone when you rap it because it
experiences very little internal friction."
You asked about a picture, but this is a discussion of an action
event, so here's a video showing how to make a wine glass "sing":
One more thing: in researching your question, I learned that Benjamin
Franklin, back in 1761, designed a musical instrument using these
principles which he called the Armonica. Unfortunately, Franklin's
instrument was made of lead glass which evidently caused neurologic
damage to many of those who played it. In 1982, an inventor named
Gerhard Finkenbeiner successfully recreated Franklin's armonica
(thankfully) using lead-free glass. You can learn more about this
instrument here: http://www.crystalmusic.com/glassarmonica.html
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Thanks for your very interesting question,
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